Compositing video layers in 3D space
You could perform the keying in the compositor scene. Create a 3D scene to reproduce the depth of the street shot. Then place UV unwrapped cards in that scene, each card could have a layer pass.

Back in the compositor apply the alpha keyed image to the 3D scene layer using UV distort. It should scale and map the people to the card.

Here is an example I made at the forum http://blenderartists.org/forum/showthread.php?332091-Card-Compositing-to-simulate-depth